Exactly like a NTFS drive can have mountpoints that are actually other drives, the Registry on disk is made by several files, at least: SAM SECURITY SYSTEM

Exactly like a NTFS drive can have mountpoints that are actually other drives, the Registry on disk is made by several files, at least: SAM SECURITY SYSTEM SOFTWARE NTUSER.DAT and since Vista BCD The whatever is called the Registry when virtually assembled together is accessed as if it was a filesystem on a single file, is not in itself a "file", it is something else, or if you prefer it seems like being a monolithic file but it is backed by a number of separated files.